there are loads of PSU's on the market, some are non modular the you have semi modular and full modular personaly i think full modular is better. Then you have different watts and 12v rails with diffrent amps so my question is is it better to have multiple 12v rails or a single 12v rail??

In theory it's actually better to have multiple rails, but for a very long time only a few PSU manufacturers implemented them properly. (ie with multi GPU setups it was easy to overload a rail, causing all sorts of issues) But in practice it's simply easier and more practical to have a single rail design unless you're running an very high wattage load. (a lot more than ~850W) With 1kW+ units it becomes a *theoretical* safety concern when you have a single 12V rail capable of ~100A - any kind of failure or short circuit would simply be exponentially more catastrophic.

I think there are many types of PSU on the market because there's so many configurations of high-end computers as well... Since you're talking about a modular PSU, I assume mid to high-end computer since you normally can't get ~250W modular PSUs.

ilya explained the multiple vs single rail quite well, so I'll chime in on the modular.

For modular vs. non-modular, you can save money with non-modular (e.g. Seasonic S series vs. M series) and you don't have to worry about a loose connection on the PSU end. Note getting such a loose connection is rare but it does happen. If you have a cramped case or bad cable management, then you'll want modular unless the added length from the modules will give you clearance issues.

In any case, before buying a power supply, look at a good review on it that FULLY loads the PSU with a load tester and measures with an oscilloscope. You can have a pretty-looking single-rail fully modular PSU but it's no good if it smokes and damages half the components it was connected to in the process.

__________________"The computer programmer says they should drive the car around the block and see if the tire fixes itself." [src]